FIG. 1 shows a schematic diagram of one embodiment of a typical prior art domino logic circuit 100. As seen in FIG. 1, prior art domino logic circuit 100 included a first supply voltage 101, typically Vdd, coupled to a source, or first flow electrode 111, of a PFET 110 and a source, or first flow electrode 121 of a PFET 120, also known as a keeper transistor. The signal CLK was coupled to a control electrode or gate 115 of PFET 110 and a control electrode or gate 135 of an NFET 130. A drain, or second flow electrode 113, of PFET 110 was coupled to a first node 190. A drain, or second flow electrode 123, of PFET 120 was also coupled to first node 190 and an input terminal 107 of an inverter 105. An output terminal 109 of inverter 105 was coupled to a control electrode or gate 125 of PFET 120 and a prior art domino logic circuit out terminal 151.
Node 190 was coupled to an input terminal 102 of a logic block 103. Logic block 103 was comprised of any one of numerous types of logic and/or circuitry used in the art including various logic gates, logic devices and circuits such as transistors, inverters and other logic functions, both simple and complex, well known to those of skill in the art, and too numerous to list comprehensively herein. Logic block 103 also included inputs at input terminals 104 and an output terminal 108. Output terminal 108 of logic block 103 was coupled to a drain, or first flow electrode 131 of NFET 130. A source, or second flow electrode 133 of NFET 130 was coupled to a second supply voltage 106, typically ground.
For illustrative purposes specific embodiments of prior art domino logic circuit 100 were shown with specific transistors. However, the NFETs and PFETS shown in the FIG. 1 can be readily exchanged for PFETs and NFETs by reversing the polarities of the supply voltages or by other well known circuit modifications.
Prior art domino logic circuit 100 had two modes, or phases, of operation; a pre-charge phase and an evaluation phase. In one embodiment of prior art domino logic circuit 100, in the pre-charge phase; the signal CLK was low or a digital “0”. Consequently, PFET 110 was conducting or “on”; PFET 120 was on and NFET 130 was off, thereby isolating logic block 103 from second supply voltage 106. In addition, during the pre-charge phase, first node 190 was high, or a digital “1”, and this state was reinforced by PFET 120 being in the on state. In addition, during the pre-charge phase, prior art domino logic circuit output terminal 151 was low or digital “0”.
In the following discussion, assume that in the previous cycle, there was a path 191 from node 190 to second supply voltage 106 through logic block 103. In the evaluation phase, the signal CLK was high or a digital “1”. Consequently, PFET 110 was not conducting or “off”; PFET 120 was on; and NFET 130 was on, thereby providing logic block 103 a path to second supply voltage 106. In addition, during the evaluation phase, first node 190 was low, or a digital “0” and prior art domino logic circuit output terminal 151 was high or digital “1”.
Prior art domino logic circuit 100 functioned reasonably well in either low speed environments or low noise environments, however, prior art domino logic circuit 100 did not perform well in high speed and high noise applications. This was because, with prior art domino logic circuit 100, the transition from the pre-charge phase to the evaluation phase involved an inherent problem regarding first node 190 and PFET 120. This problem arose because, as discussed above, in the pre-charge phase, first node 190 of prior art domino logic circuit 100 was held at a digital “1” and prior art domino logic circuit output node 151 was a digital “0”, which reinforced the digital “1” on first node 190 by keeping PFET 120 on. At the transition from pre-charge to evaluation phase, the signal CLK goes to a digital “1” and NFET 130 is turned on, consequently, logic block 103 is provided with a path to second source voltage 106. If, as was often the case in many instances and types of logic used in logic block 103, logic block 103 also provided a path to NFET 130 at this time, i.e., logic block 103 was also “on”, then a path 191 from first node 190 (FIG. 1) to second supply voltage 106, typically ground, through logic block 103 and NFET 130 was established. Once path 191 was established, first node 190 should have dropped to a digital “0” as rapidly as possible to avoid delays in operation of prior art domino logic circuit 100. However, in this same time frame, PFET 120 was still transitioning to the off state, i.e., was still on, and this meant that PFET 120 was still trying to hold first node 190 at first supply voltage 101, i.e., at a digital “1”. Consequently, in prior art domino logic circuit 100 there was an inherent “fight” between first node 190, trying to discharge to “0” and PFET 120 trying to hold first node 190 at “1” during the transition between pre-charge and evaluation. This fight resulted in a significant delay in the operation of prior art domino logic circuit 100.
To try and minimize this effect, i.e., the delay, resulting from the “fight” between first node 190, trying to discharge to “0” and PFET 120 trying to hold first node 190 at “1” during the transition between pre-charge and evaluation, most circuit designers employed a PFET 120 with the smallest possible channel dimensions, i.e., PFET 120 was intentionally made small, and therefore weak, so that PFET 120 would hold node 190 high for as short a time as possible. In other words, PFET 120 was made weak and small so it would lose its fight with first node 190 quickly. Unfortunately, this solution had significant drawbacks. In particular, by making PFET 120 small, the noise immunity of prior art domino logic circuit 100 was compromised and this could lead to total failure of prior art domino logic circuit 100 in high noise environments.
Employing a weak PFET 120 in prior art domino logic circuit 100 was particularly problematic in instances where logic block 103 did not provide a path to NFET 130 and second supply voltage 106. In these instances, first node 190 must remain high. However, if noise was introduced at input terminals 104 of logic block 103, this noise could cause logic block 103 to provide a temporary path to NFET 130 and second supply voltage 106. In this case, first node 190 could discharge to ground, i.e., first node 190 could go low in error, and there was no mechanism to ever bring first node 190 back to high or digital “1”. Consequently, under these circumstances, prior art domino logic circuit 100 would fail unrecoverably.
As a result of the situation discussed above, designers of prior art domino logic circuit 100 were constantly involved in a balancing act between minimizing the size and strength of PFET 120, to increase speed of prior art domino logic circuit 100, and increasing the size and strength of PFET 120, to make prior art domino logic circuit 100 more robust and noise immune. The result was that prior art domino logic circuit 100 functioned reasonably well in either low speed environments or low noise environments, however, prior art domino logic circuit 100 did not perform well in high speed and high noise applications.
What is needed is a method and apparatus for creating an improved domino logic circuit that is capable of operation in both high speed and high noise environments.